For the reduction of pollutants, in particular for the reduction of nitrogen oxides, various methods have become established in the case of which reducing fluids (gases or liquids) are introduced into the exhaust system of an internal combustion engine.
In particular, for the reduction of nitrogen oxides, SCR (Selective Catalytic Reduction) has proven successful, in the case of which nitrogen oxides (NOX) contained in the oxygen-rich exhaust gas are selectively reduced to form nitrogen and water, with the aid of ammonia or a corresponding precursor substance that can be converted into ammonia. Aqueous urea solutions may be used. The urea solution is hydrolyzed by way of hydrolysis catalytic converters or directly on the SCR catalytic converter to form ammonia and carbon dioxide. For this purpose, the urea solution is carried in one or more tanks and is conducted by way of a delivery device to a dosing system which introduces the urea solution into the exhaust-gas stream upstream of the hydrolysis catalytic converter or upstream of the SCR catalytic converter.
As a reducing agent, an aqueous urea solution with a urea content of 31.8-33.2 percent by weight, which is marketed under the trademark “AdBlue” may be used. As in the case of other reducing agent solutions, however, this urea solution is also afflicted with a relatively low freezing point (−11.5° C.). Once frozen, a reducing agent can no longer be delivered to the catalytic converter. This results, in particular in winter, in potential failure of the exhaust-gas aftertreatment system, as a result of which inadmissibly high emissions of harmful exhaust-gas components can occur. Furthermore, the volume of the aqueous urea solution increases by approximately 10% when it freezes. This can result in damage to the individual components of the SCR exhaust-gas aftertreatment system.
The EC Regulation no. 692/2008 prescribes that, for vehicles which require a reagent for their exhaust-gas aftertreatment system, said system must perform its emissions reduction function even at low ambient temperatures. This also includes measures for preventing complete freezing of the reagent over a parking duration of up to 7 days at −15° C. in the case of a tank fill level of 50%.
If the reagent has frozen, it must be ensured that said reagent is available for use, in order that the emissions reduction system can operate correctly, within 20 minutes after starting of the vehicle in the case of a temperature of −15° C. having been measured in the reagent vessel.
For the starting of operation and correct functioning of an exhaust-gas aftertreatment system of said type, it is therefore necessary for the tank in particular to be equipped with heating systems which melt the frozen liquid in the tank and change said reagent into a liquid, that is to say flowable, state of aggregation.
Systems are already known from the prior art which encase liquid tanks in motor vehicles with heating foils or heating mats in order to thaw the frozen liquid in the tank. Heating foils or heating mats arranged on the outside on the tank wall however exhibit low efficiency, because a major part of the heat generated is not conducted into the tank so as to melt the frozen liquid but is dissipated into the tank surroundings as power losses. Therefore, an increase either of the heating temperature and/or of the activation time of the heater is necessary in order that the tank, or at least that part of the tank from which the reducing agent is extracted, is heated such that the reducing agent is thawed, or thaws, adequately quickly and thus remains in the flowable state of aggregation (EP 1 767 417 A1, DE 10 2007 005 004 A1).
Furthermore, it is known from the prior art for not only the tank but also at least a part of the reducing agent lines, the reducing agent pump, the reducing agent filter and/or the reducing agent injector may be heated (DE 10 2008 061 471 A1, WO 2006/90182 A1, EP 2 133 527 A1).